The principal aim of this protocol is to establish the kinetic basis of primary human HDL deficiency, i.e. decreased production, increased catabolism, or possibly both. It is possible that different kinetic phenotypes will be identified in some patients compared to others. The kinetic phenotype will be correlated with mutations discovered in the genes or regulatory elements of genes for HDL proteins or of gene products thought to be involved in the de novo generation of HDL. A second goal is to study the role of prebeta-I HDL, a discrete HDL molecular subspecies of a possible precursor of HDL. The kinetics will be studied using stable isotope-labeled amino acids. Enrichment will be measured by mass spectrometry. The GCRC will be needed to regulate the diet over a one-month period before the isotope study and to facilitate the infusion and sample collection.